Does anyone have a suggestion for adjusting mic gain on the FT-60? I need to increase it to use a Heil HTHY headset.

As I said in my other reply, make sure you are talking into eather end of the capsule, not the side, and have it about an inch from your mouth, if not closer. Make sure the plug is firmly seated. I haven’t had a single problem with several HTH headsets for yaesu and Kenwood HT’s here.